Although the major task of the visual system is to build a spatial map of the environment, it is also able to detect the wavelength of light. To achieve color vision, animals compare the inputs of at least two spectral types of photoreceptors (PRs). In Drosophila, color vision is achieved by the two inner PRs R7 and R8 which express different rhodopsins (rhs) and project to close targets in the medulla part of the optic lobe. Three classes of ommatidia with different types of inner PRs exist. In p ommatidia, R7 contains the UV-Rh3 while R8 contains blue-Rh5. In y ommatidia, R7 contains UV-Rh4 and R8 green-Rh6. This results from a stochastic decision made in R7 to express rh4 and to exclude rh3. A signal is then sent from the rh3-expressing R7 to induce rh5 in R8. By default, the other R8 express rh6. A third class of ommatidia, located at the dorsal rim area (DRA), express Rh3 in both R7 and R8. These PRs measure the vector of light polarization, contributing to the "compass" of the fly. In the following aims, we propose to investigate how the distinction between the three classes of ommatidia is achieved: 1. Stochastic decision for the choice of color photoreceptors: After their initial recruitment, the inner PRs differentiate sequentially to acquire their specific subtype. We will study the steps that include control of inner PR differentiation by spalt, of R7 fate by prospero and of R8 fate by senseless. We will investigate how spineless controls the choice between p and y ommatidia by inducing rh4 expression in yR7, and how its expression is regulated. 2. Localized control of polarized light sensitive photoreceptors: We will continue our investigations of the signaling events that allow the expression of homothorax in the DRA. We will study how hth and other effectors control rh gene expression and adjust the morphogenesis of these PRs to the special task of polarized light detection. 3. Transcriptional expression of rhodopsin promoters: We will continue our dissection of the rh promoters as critical tools to understand the genetic regulations leading to PR subtype specification. We will focus on the repression mechanisms that allow rh6 to be restricted to yR8 and on the regulation ofrh3 in the DRA. We will also study how rh4 reflects the stochastic choice resulting from spineless expression, and how rh5 is controlled. The regulative network will then be reconstructed in cell culture. 4. Enhancer trap screen for genes expressed in subsets of photoreceptors: We will continue the investigation of genes that were identified in an enhancer trap screen. We will also utilize a new type of transposable element (piggyBac) to identify genes that have not been targeted by P-elements. An EP-type over-expression screen to identify further regulators will also be continued. Finally, we will use a bioinformatics approach to search for promoters sharing regulatory elements with PR specific genes. [unreadable] [unreadable] [unreadable]